Heretofore, there has been inadequate analysis and understanding of the very complex problem of threaded fasteners loosening and causing failures, often with disastrous results. Threaded fasteners are the most pervasive systems used in many critical hardware assemblies. There are four major requirements for a threaded fastener system to be reliable. These are: proper clamping force or preload; screw locking (anti-rotation); adjustment for stress relaxation and creep to maintain proper clamping force; and, screw, nut or head restraint to prevent relative motion between the screw and the clamped structure. A great deal of investigation and implementation has been directed to the first two requirements so that a working understanding is being used to control their variable characteristics. For such a simple and widely used system, controlling all the variables of the first two characteristics is not enough to produce a reliable system, as screws still loosen and fall out. Attention is directed to the last two requirements and the reasons why the system failures will continue to occur unless a working knowledge of their characteristics is pursued and new hardware and processes utilized.
Adjustment for stress relaxation and creep to maintain a proper clamping force is of major importance to prevent system failures. Stress relaxation is any joint thickness change due to plastic deformation above the yield strength of the metal or other material, and creep is plastic or permanent joint thickness change due to stress, temperature and/or time related conditions that occur below the yield strength. As to screw, nut or head restraint to prevent relative motion between the screw and the clamped structure, relative motion due to environmental forces will apply a torque to a screw head or nut that will be oscillatory and persistant. The torque to tighten the screw is resisted, while the torque to loosen the screw is not resisted. This ratchet action will cause a screw to loosen and fall out.
In an idealized screw joint, the only spring force to keep the joint tight is the tension created in the screw itself, see the exaggerated illustration of FIG. 1, for example, showing the screw stretched. The introduction of other spring forces is thought to seriously degrade joint performance because of possible screw fatigue or overloading. All screw systems are designed with this idealized system in mind. Very seldom do the actual hardware and fabrication processes produce the idealized system.
Screw joints frequently have a lot of springs beside the screw itself, both inside and outside of the clamped joint sandwich. If these springs do not change, then their forces produce a joint that operates reliably, even when joint characteristics change. These springs will not degrade screw performance in fatigue or overload environments because the actual screw loads are generally very low. Screws and nuts, nevertheless, can be loosened by vibrations at their natural frequencies, without application of external forces.
Standard hardware used in a typical screw system, together with the standard machining processes utilized to fabricate the support structures, all work together to produce at least seven spring systems in the screw joints such as shown in FIG. 2, for example. Some of the hardware configurations depicted produce springs that can easily change when subjected to large screw forces or environmental conditions that promote stress relaxation and creep.
Creep is stress, temperature and time related, and it is that characteristic of a material that results in permanent deformation even when the material is stressed below its yield point. Any creep that occurs reduces the joint thickness and thereby reduces the screw clamping load. The actual screw-spring stretch is so small that very little joint thickness change needs to occur to produce a loose assembly. Also, any environmental forces that produce clamped plate relative motion at the screw head or nut can cause an unscrewing process to occur. These motions can be large or as small as a millionth of an inch.
A comparison of screw-joint spring systems reveals that all have very low spring rates when measured against the screw itself. These low rates produce very small serial forces that can have large reductions with small joint thickness changes. Joint thickness changes will occur due to stress relaxation and creep wherever the screw load is supported by very small areas. This condition can occur in all the spring systems including the screw itself. Causes of this thickness change are many; they include: large screw forces; burrs; shock; rough surface texture; temperature changes; plating crushing and wear; material stress relief due to vibration; plate ringing during vibration; and, poor geometry, including hole edge mounding due to machining forces, potato-chip surfaces typical of sheet metal parts, and non-coplanar support surfaces. The variety of the listed causes are numerous. Experience, however, indicates that screw systems, as a whole, operate reliably. The configurations depicted in FIG. 2, with environmental inputs, unfortunately, do cause unpredictable and seemingly random failures. It is because of these failures that changes in fabrication processes and hardware configurations become necessary in critical situations where a very high degree of confidence that the system operate reliably is required.
Joint thickness changes and the attendant problems read to be minimized. Part fabrication processes that produce flat mating surfaces are important. This can be done by making surfaces near screw holes flat after all drilling, deburring, plating and insert assembling by any of a variety of processes including: surface grinding, spot facing, lapping, surface crushing, and single-point skin cutting. However, a new approach is to eliminate the effects of stress relaxation and creep without any changes in existing fabrication techniques and assembly processes. It has been found that this can be accomplished with a low-rate high-force spring placed under a screw head or under a nut to provide a continuously large force during and after joint thickness changes due to stress relaxation, creep or wear. Prior devices, such as a Belleville washer, wave washer, extra duty spring-lock washer, or any one of several other clever spring-type systems have been considered for this purpose. None of these aforementioned configurations, however, are completely suitable because: most require an additional flat washer; some have relatively little deflection, most do not exert high required loads; some are too large; some are too costly; and, all require the rigorous use of torque measuring devices and processes.
A new device which has characteristics that meet most of the necessary requirements for a threaded connection to prevent loosening, as discussed above, is an inexpensive graduated-load spring washer system for screws and threaded fasteners, hereinafter described below.